1. Field of the Invention
This invention relates to a rotary dobby for the operation of the heddle frames installed on a loom and a weaving loom and to a loom provided with such a dobby.
2. Description of the Related Art
It is known that in rotary dobbies the vertical movement of the heddle frames is provided by oscillating components that can be constituted, depending on the case, by connecting rod-arm assemblies or by roller-bearing arms; these oscillating parts are driven by actuating elements in the form of an eccentric gear in the first case or of a cam in the second one. These actuating elements are mounted on a main shaft of the mechanism that is actuated by an intermittent rotary movement and, at the time of each stoppage, as a matter of fact at all the half turns of the above-mentioned shaft, the reading-in device must interlock the actuating element either with the shaft, in order to drive the oscillating part, or with a stationary point in order to effect the angular immobilization of the latter; this interlocking must be effected at each of heddles of the dobby, that is to say of the actuating unit associated with each heddle frame and depending on the design or weave to be obtained on the loom during the weaving process.
This selective interlocking is generally obtained by means of a cotter or catch shaped movable coupling element subjected to the action of two pivoting arms arranged on one and the other sides of the shaft in order to actuate this moveable element to its two stop positions, being each pair of pivoting arms controlled by the reading-in device of the dobby.
In patent application FR-A-2 540 524 was disclosed a rotary dobby for looms in which a plate joined to each heddle frame is comprised of two diametrically opposed notches suitable to interact with the catch of two pivoting arms that are controlled by the reading-in device. These two notches are of different shape because one of them must be relatively deep in order to provide the plate with a perfectly precise angular position and then ensure an adequate holding for as long as necessary. On the other hand, the other notch is of reduced depth and is provided with lateral walls which are wide open and that run parallel to the chamfers of the extremities of the sides of the catch of each pivoting arm, so that the arm's catch can be automatically driven with the rotation of the plate, without the actuating of the reading-in device upon the arm. In this second case, one talks about a "passive" engaging of the catch in the notch because, even if the elastic engaging takes place in the same manner in both notches, the catch of the second arm can be passively pushed back.
Tests have shown that such an arrangement functions in a satisfactory manner, but the catch of each arm must be relatively elongated because the bearing surfaces between the first and the second notches of the plate have height that are added up dimensions or cumulative in the direction of the release of the catch with respect to these notches. This means that the angular movement of each of the pivoting arms must have a relatively large amplitude in order to allow the release of the two bearing surfaces provided on each notch of these arms. The necessary power for such an angular movement is high, hence a relatively high energy consumption of the dobby in question.
Furthermore, these large-amplitude angular movements are carried out through strong impacts on the pivoting arms, which cause a premature wear and tear of the components of the dobby of known design, unless these components are of a particular strong construction. Further, these impacts are noise generators, which is an irritant as regards the operators whose work stations are in close proximity of the looms provided with the known type of dobbies.
Lastly, the pivoting of the arms is not instantaneous and the wider its amplitude the longer it takes. This is because wide amplitude pivoting of the arms is of such nature that it limits the speed of the known dobbies.